Lithium ion secondary batteries have high capacity and high energy density, and their size and weight reduction can be easily achieved. Therefore, lithium ion secondary batteries are widely used as a power source for portable small electronic devices, including, for example, mobile phones, personal digital assistants (PDAs), notebook personal computers, camcorders, and portable game devices. In a typical lithium ion secondary battery, a positive electrode containing a lithium cobalt compound as the positive electrode active material, a negative electrode containing a carbon material as the negative electrode active material, and a separator made of a polyolefin porous film are used. Such lithium ion secondary batteries have high battery capacity and high output, as well as excellent charge and discharge cycle performance, and relatively long durable life. However, under current situations where portable small electronic devices are becoming multifunctional and extension of continuously usable time is demanded, lithium ion secondary batteries are required to have even higher capacity.
For achieving even higher capacity lithium ion secondary batteries, for example, development of a high capacity negative electrode active material is in progress. Alloy-based negative electrode active materials that are capable of forming an alloy with lithium and reversibly absorb and desorb lithium are gaining attention as a high capacity negative electrode active material. Known alloy-based negative electrode active materials include, for example, silicon, tin, oxides of these, nitrides of these, and compounds and alloys containing these. The alloy-based negative electrode active materials have a high discharge capacity. For example, Japanese Laid-Open Patent Publication No. 2002-83594 (in the following, referred to as “Patent Document 1”) mentions that silicon has a theoretical discharge capacity of about 4199 mAh/g, which is about eleven times the theoretical discharge capacity of graphite, which has been used as the negative electrode active material.
The alloy-based negative electrode active material is effective in terms of achieving a high capacity lithium ion secondary battery. However, for realizing practical use of a lithium ion secondary battery containing the alloy-based negative electrode active material, there are several problems to be solved. For example, it is very important to ensure safety of a lithium ion secondary battery containing an alloy-based negative electrode active material. Not only in lithium ion secondary batteries, in secondary batteries, heat generation may occur as a result of overcharging, an internal short circuit, inclusion of foreign matter, and the like, although in a very rare case. In a lithium ion secondary battery containing an alloy-based negative electrode active material, a large amount of heat generation may possibly occur due to the high capacity of the alloy-based negative electrode active material. Therefore, it is required that, as one safety condition, heat radiation is carried out immediately to avoid high temperature in the battery even if the heat generation occurred by some chance.
For example, there has been proposed in Japanese Laid-Open Patent Publication No. Hei 5-166500 (in the following, referred to as “Patent Document 2”) a battery having a heat radiating fin. The battery in Patent Document 2 includes a wound-type electrode assembly in which a rectangular positive electrode, a solid electrolyte, and a rectangular negative electrode are piled up to form a stack, and the obtained stack is spirally wound with one short side thereof as the center. The positive electrode includes a rectangular positive electrode current collector with a heat radiating fin provided at the end of the long side thereof, and a positive electrode active material layer formed on a portion of the surface of the positive electrode current collector other than the portion where the heat radiating fin portion is provided. The negative electrode includes a rectangular negative electrode current collector with a heat radiating fin provided at the end of the long side thereof, and a negative electrode active material layer formed on a portion of the surface of the negative electrode current collector other than the portion where the heat radiating fin portion is provided. The heat radiating fin of the positive electrode current collector has through holes formed extending through the thickness or cut-out sections, and projects outwardly from one end of the wound-type electrode assembly in a spirally-wound form. The heat radiating fin of the negative electrode current collector also has through holes formed extending through the thickness or cut-out sections, and projects outwardly from the other end of the wound-type electrode assembly in a spirally-wound form.
As described above, the heat radiating fins in Patent Document 2 are in spirally wound form, and the entire heat radiating fins are positioned in close proximity; therefore, heat radiation efficiency is insufficient. Particularly, at the long side of the positive electrode current collector and the negative electrode current collector where the heat radiating fin is not provided, heat radiation efficiency declines. For example, at the long side of the positive electrode current collector where the heat radiating fin is not provided, the heat radiating fin of the negative electrode current collector is present. However, because the solid electrolyte, which does not have good thermal conductivity, is present between the positive electrode and the negative electrode, thermal conduction efficiency between the positive and negative electrodes is insufficient. Therefore, it is difficult to diffuse heat to the outside by the heat radiating fin of the negative electrode current collector at the long side of the positive electrode current collector where the heat radiating fin is not provided. Thus, it is difficult to diffuse the heat of the entire battery in a substantially uniform manner simply by providing a heat radiating fin in a spirally wound form.
Thus, in the technique of Patent Document 2, an excellent electrical conductor is connected to a projecting end portion of the heat radiating fin. However, even if such a measure is taken, heat radiation efficiency at the long side where the heat radiating fin is not provided does not sufficiently improve. Additionally, use of the excellent electrical conductor, which is an extra component for a battery, complicates the battery structure, and may possibly result in a higher defective rate in the manufacturing process and an increased production cost. Also, attachment failure may be an additional cause of heat generation.
Although Patent Document 2 describes a battery in accordance with the document as a stack-type thin battery, the battery actually made in the document is a wound-type electrode assembly; therefore, the battery of Patent Document 2 is not the stack-type battery.
There has been proposed in Japanese Laid-Open Patent Publication No. Hei 9-134731 (in the following, referred to as “Patent Document 3”) a solid electrolyte battery including a rectangular flat plate electrode assembly, and a heat radiating fin (cooling fin) projecting from one of the sides of the electrode assembly toward outside of the electrode assembly. In this battery as well, the heat radiating fin is provided as an extended portion of the current collector. Also, it is implied in the document that a plurality of heat radiating fins are provided in one current collector, because the current collector is described as being projected at at least one side. However, in the configuration of Patent Document 3, a frame body includes a plurality of electrode assembly attachment apertures, and a plurality of electrode assemblies are attached to the frame body. In order to provide the plurality of heat radiating fins, many openings have to be provided in the frame body for allowing the heat radiating fins to penetrate, which result in a decrease, for example, in the mechanical strength and ability to retain the electrode assembly of the frame body. Therefore, it is technically difficult to provide a plurality of heat radiating fins in one current collector. Therefore, Patent Document 3 only discloses in detail a configuration in which one heat radiating fin is provided in one current collector.
As described above, providing only one heat radiating fin leads to poor heat radiation efficiency. Moreover, because the heat radiating fins of the electrode assemblies are disposed in close proximity in Patent Document 3, heat radiation efficiency is reduced even more. For this reason, the heat radiating fins of the electrode assemblies are disposed in close proximity and are connected electrically, but the heat radiation effect is insufficient.